Shock wave processing techniques of materials have received much attention because of their unique ability to synthesize and compact difficult-to-consolidate metallic and non-metallic powders, especially certain non-oxide ceramics such as boron nitride (BN), silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) Synthetic cubic boron nitride (C-BN) has a hardness of 66 to 75 GPa on the micro-Vickers scale, only second to diamond (120 GPa), and a very high compressive strength of 4.15 to 5.33 GPa. It is potentially a very useful material when fabricated into cutting tools. Moreover, as a structural material it could be used in high temperature-high pressure environments.
Silicon nitride and silicon carbide whiskers have tensile strengths (14 GPa and 21 GPa, respectively) higher than steel (13.3 GPa) and very stable at temperatures as high as 1400.degree. C. in an oxygen-rich environment. It is desirable to consolidate cubic boron nitride with silicon carbide whisker (SCW) or silicon nitride whisker (SNW) to produce a composite which is both hard and tough.
Although shock wave consolidation of super hard materials has been studied since the 1950's, only modest progress in achieving complete consolidation has occurred.
A series of attempts have been made to consolidate cubic boron nitride powder, employing static high pressure and high temperature. Two dense polymorphs of BN can be produced directly from the low pressure phase, graphite-structured boron nitride (g-BN, density 2.29 g/cm.sup.3). These are the zincblende structure (C-BN, density 3.487 g/cm.sup.3) and wurtzite structure (W-BN, density 3.454 g/cm.sup.3). The thermophysical and thermochemical properties of C-BN are similar to those of diamond. Both diamond and C-BN convert to the graphite structure at temperatures of .apprxeq.1700 K in vacuum. However, unlike diamond, C--BN remains stable up to about 1300.degree. K. in the presence of oxygen. Complex phase transition relationships have been found to exist between different forms of BN under various shock loading conditions. A main problem in direct shock consolidation of C--BN without a binding material is that a fraction of the C--BN converts into an amorphous low density form due to the high temperatures in the post-shock release process from very high pressures.
Moreover, it appears that the high thermal conductivity of C--BN (.apprxeq.8 watt/cm deg, which has a value approximately twice that of pure copper) is another problem in its dynamic consolidation. This is because the surface layer of melted material, resulting from preferential energy deposition via friction and plastic deformation in grain surface, is quenched so rapidly via radiation and Fourier conduction into the grain interior, that substantial surface melting does not occur. As a result, particles become almost isothermal after shock passage.